In motor vehicles with internal combustion engines, in particular diesel engines, it is necessary, owing to the ever more stringent legal exhaust-gas limit values, to further reduce inter alia air pollutants such as for example nitrogen oxides (NOx), in the exhaust-gas flow. A widely used method which is applied here is catalytic reduction, that is to say the so-called SCR method (“Selective Catalytic Reduction”). Here, during the operation of an internal combustion engine, in particular of a diesel engine, a liquid reducing agent is delivered by means of a pump from a storage vessel to a dosing module with an injection nozzle in the region of a catalytic converter in the exhaust pipe. As reducing agent, use is generally made of a 32.5% urea-water solution, which is marketed under the trade name “AdBlue®”. In this connection, it must be taken into consideration that said urea-water solution freezes at a temperature below −11° C., and thermally decomposes above 60° C., such that, in particular for winter operation at low temperatures, heating devices must be provided in order to ensure the necessary ice pressure resistance.
The prior art discloses numerous devices for storing the urea-water solution required for the SCR method in diesel engines and for supplying said reducing agent to the exhaust-gas aftertreatment systems. DE 10 2008 041 723 A1 discloses a tank for storing a liquid active substance. One end of a flexible suctioning lance is surrounded, in the vicinity of the tank base, by a heating means, wherein the suctioning line additionally has heating wires on the inside and/or on the outside. The entire arrangement is situated in a slosh pot in order to permit attitude-independent suctioning of the reducing agent. Laterally on the slosh pot there is situated a fill level sensor. The slosh pot and the fill level sensor are arranged within a tank. An electrical connection line of the fill level sensor and the suctioning line are provided with a fabric hose as protection against mechanical abrasion. Since the fill level sensor does not have a direct heating device, it cannot be ruled out that said fill level sensor will freeze, in particular in the event of frost temperatures prevailing over a long period of time. DE 10 2008 041 805 A1 discloses a further tank arrangement. The ice pressure resistance is realized in said tank by an air cushion situated beneath the tank cover, which air cushion is formed even in the case of a maximum fill level. To generate the air cushion, a cubical or hollow cylindrical space delimitation means is for example provided beneath the tank cover, which space delimitation means is open toward the tank base. During the filling of the tank, therefore, an air cushion forms in the space delimitation means because the air cannot escape laterally out of the space delimitation means. In the event of freezing of the tank contents, said air cushion is compressed, whereby damage to the tank is prevented. In the event of extreme oblique positions of the tank, however, the ice pressure resistance cannot be ensured because the air cushion can at least partially escape out of the space delimitation means.
DE 10 2007 059 853 A1 relates to a device for the measurement of a fill level of a liquid in a vessel. For the fill level measurement, an ultrasound transducer is arranged in the region of the tank base, the ultrasound beam of which runs parallel to the tank base. By means of a diverting element on the tank base, the ultrasound beam is diverted through 90° and directed from below to the liquid surface through a measurement pipe, is reflected back from there and is conducted by the diverting element back to the ultrasound transducer. By means of evaluation electronics connected downstream of the ultrasound transducer, the level of the liquid level in the tank can be determined from the reflected ultrasound signal. Said device for measuring a vessel surface level however requires an extremely large amount of technical outlay.
DE 10 2007 028 147 A1 furthermore discloses a device for detecting the fill level of a liquid in a vessel, said device being realized with a pressure sensor. The pressure sensor is arranged in the region of the vessel base and is surrounded by a cylindrical housing, the height of which corresponds approximately to the maximum fill level. The housing is penetrated by a multiplicity of small compensation openings which impart a throttling action to the liquid flowing through, such that substantially only the hydrostatic pressure of the liquid acts on the sensor. In this way, the output signal of the sensor has a high degree of temporal constancy. Excessive tilting of the vessel or freezing of the liquid at least in regions, however, can under some circumstances influence the measurement result.
It is therefore an object of the invention to provide a device for reliably supplying a reducing agent to an exhaust-gas aftertreatment system of a diesel engine, which device ensures a reliable discharge of the reducing agent to the exhaust-gas aftertreatment system under all ambient conditions and operating states that normally arise in motor vehicles, which device simultaneously permits precise detection and monitoring of the reducing agent level, which device is of simple construction, and which device, owing to a low space requirement or a compact design, can be easily integrated into a motor vehicle.